As one can ascertain, there are numerous line codes that are specifically designed to not contain DC energy and thereby be unaffected by DC removal. One example of such a line code is bi-polar coding which solves the DC wander problem by using three levels to encode binary data. Specifically, a logic zero is encoded with zero voltage while a logic "1" is alternately encoded with positive and negative voltages. Hence the average voltage level is maintained at zero to eliminate DC components in the signal spectrum. Since bi-polar coding uses alternate polarity pulses for encoding logic "1's", it is also referred to as "alternate mark inversion" (AMI). A mark is a term arising from telegraphy to refer to the active or "1" state of a level encoded transmission line.
Bi-polar coding is the basic line coding procedure used in many telephone systems as, for example, by T1 lines in the telephone network. Essentially, rather than use full pulse periods, T1 lines use a 50% duty cycle pulse to encode each logic 1. 50% duty cycle pulses were selected to simplify timing recovery in the regenerative repeaters of a T1 line. A great deal of work has been done in regard to bi-polar codes. Because a bi-polar code uses alternating polarities for encoding "1's", strings of "1's" have strong timing components. However, a string of zeros contains no timing information and therefore must be precluded by the source. Alternate mark inversion systems are widely employed in a telephony. However, in most systems it is desirable to utilize or convert such a signal into signals suitable for operation with other types of logic structures. For example, a widely employed logic structure utilizes TTL technology (transistor-transistor logic). TTL integrated circuits have gained wide acceptance and have been used for many years. This family of integrated circuits reached broad popularity because of the compromise between speed and power. It is desirable to utilize such logic circuitry in telephone systems and other systems as well because the circuits are extremely reliable, have relatively low power consumption, and operate at high speeds with low propagation delay.
It is therefore an object of the present invention to provide an efficient and novel converter circuit which enables one to convert an AMI signal into unipolar output signals.
It is a further object of the present invention to convert an AMI signal into signals which can be employed with TTL logic circuits.